DTC P1639: 5-Volt Reference 2 Circuit: Description

WARNING: This page is about a different car, the 2002 Oldsmobile Aurora. However, it is still accessible from the selected car via links, so may be relevant.

The PCM uses the 5-volt reference 2 circuit as a sensor feed for the following sensors:

The AC pressure sensor.
The Fuel Tank Pressure (FTP) sensor.

The PCM monitors the voltage on the 5-volt reference 2 circuit. If the voltage is outside an acceptable range, the PCM will set DTC P1639.